A so-called white Light Emitting Diode (LED) is a solid state device that converts electrical energy into light. FIG. 1 (Prior Art) is a simplified cross-sectional diagram of a white LED assembly 1. The white LED assembly is actually an assembly that includes a blue light emitting diode (LED) die 2, commonly referred to as a blue LED. The active region 3 within the blue LED die is an InGaN/GaN Multiple Quantum Well (MQW) structure. The blue LED die therefore emits relatively narrowband blue light having a wavelength in a range from 455 nanometers (nm) to about 455 nm. Blue LED die 2 is mounted in a reflector cup of a substrate 4. In addition to blue LED die 2 and substrate 4, the white LED assembly 1 includes an amount of luminescent material 5. Luminescent material 5 includes phosphor particles and a transparent binding material. The luminescent material may be generally referred to as “phosphor”, even though more accurately it includes the binding material and phosphor particles. In the example illustrated in FIG. 1, luminescent material 5 includes a mixture of three types of phosphor particles suspended in a transparent silicone binding material.
A solid black dot symbol represents a red-emitting phosphor particle. A particular red-emitting phosphor particle may, for example, absorb blue light emitted form the LED die. The blue light has a wavelength of 455 nm. The red-emitting phosphor particle then re-emits some of the absorbed energy as red light. The emission spectrum of the re-emitted light is centered at a peak wavelength of about 650 nm.
A small dot symbol having a white center represents an orange/yellow-light emitting phosphor particle. A particular yellow/orange-emitting phosphor particle may, for example, absorb 455 nm blue light from the blue LED die, and then re-emit some of the absorbed energy as yellow/orange light. The emission spectrum of the re-emitted light is centered at a peak wavelength of about 574 nm.
A small “x” symbol represents a green-emitting phosphor particle. A particular green-emitting phosphor particle may, for example, absorb 455 nm blue light from the blue LED die, and can then re-emit some of the absorbed energy as green light. The emission spectrum of the re-emitted light is centered at a peak wavelength of about 545 nm.
Some of the blue light emitted by blue LED die 2 passes all the way through the fluorescent material 5 without being absorbed by any phosphor particle and exits the LED device 1 as blue light. Another portion of the blue light emitted by the blue LED die is absorbed by red-emitting phosphor particles such that the red-emitting phosphor particles emit red light that in turn exits the LED device. Another portion of the blue light emitted by the blue LED die is absorbed by yellow/orange-emitting phosphor particles such that the yellow/orange-emitting particles emit yellow/orange light that in turn exits the LED device. Another portion of the blue light emitted by the blue LED die is absorbed by green-emitting phosphor particles such that the green-emitting particles emit green light that in turn exits the LED device. The relative amounts, types, and relative positioning of the three different phosphors within the LED device structure is such that the color spectrum of the combined light that exits the LED device appears to the human eye as being “white”. The LED device is therefore commonly referred to as a “white LED”.
FIG. 2 (Prior Art) is a simplified cross-sectional diagram of a part of white LED device 1. Each arrow in the diagram represents the path of a photon. The photon represented by arrow 6 is blue light that passes through the fluorescent material 5 and exits the LED device as blue light. The photon represented by arrow 7 is blue light that is absorbed by a red-emitting phosphor particle, that in turn emits a red light photon 8 that exists the LED device. The photon represented by arrow 9 is blue light that is absorbed by a yellow/orange-emitting phosphor particle, that in turn emits yellow/orange light 10 that exits the LED device. The photon represented by arrow 11 is blue light that is absorbed by a green-emitting phosphor particle, that in turn emits green light 12 that exits the LED device.
A phenomenon referred to as “interabsorption” can also occur. A phosphor particle can absorb light emitted from another phosphor particle. For example, a blue light photon represented by arrow 13 is absorbed by a green-emitting phosphor particle. The green-emitting phosphor particle then re-emits green light represented by arrow 14. Rather than exiting the LED assembly, the green light is absorbed by a red-emitting phosphor particle. The red-emitting phosphor particle then re-emits red light 15. Similarly, arrow 16 represents a blue light photon that is absorbed by a yellow/orange-emitting phosphor particle. The yellow/orange-emitting phosphor particle then re-emits yellow-orange light 17. Rather than the yellow/orange light exiting 17 the LED device, the yellow/orange light 17 is absorbed by a red-emitting phosphor particle, that in turn emits red light 18. Although not illustrated, other complex interabsorption events can occur. For example, light emitted from an excited green-emitting phosphor particle may be absorbed by absorbed by a yellow/orange-emitting phosphor particle.
FIGS. 3-6 (Prior Art) are diagrams of LED assemblies set forth in U.S. Pat. No. 7,250,714. In the LED assembly 19 of FIG. 3, the red-emitting phosphor 20 and the green/yellow-emitting phosphor 21 are deposited adjacent to each other with respect to blue LED die 22 such that absorption by the red-emitting phosphor of light emitted by the green/yellow emitting phosphor is reduced. In the LED assembly 23 of FIG. 4, the green/yellow-emitting phosphor 24 and the red-emitting phosphor 25 are deposited over LED die 26 as discrete layers, with the red-emitting phosphor layer 25 deposited closest to the LED 26. The green/yellow emitting-phosphor layer and the red-emitting phosphor layers may be separated by an optional transparent layer 27. In the LED assembly 28 of FIG. 5, the green/yellow-emitting phosphor 29 and the other phosphors 30 are deposited in a plurality of small regions on a blue LED die 31 as illustrated. In the LED assembly 32 of FIG. 6, small regions 33 of red-emitting phosphor are formed on the surface of a blue LED die 34. A layer of green/yellow-emitting phosphor 35 is deposited over the plurality of regions 33 as illustrated. In each of the LED assemblies of FIGS. 3-6, the arrangements of the yellow/orange-emitting phosphor and the red-emitting phosphor reduce the probability that light emitted from the green/yellow-emitting phosphor will be absorbed by the red-emitting phosphor. See U.S. Pat. No. 7,250,714 for further details.